On-board vehicular systems are usually used in automative vehicles, boats, or other installations where a self-contained electrical power network is desirable. Such systems utilize a battery when the prime mover, typically an automotive-type internal combustion engine, or the like, is not operating or only operating slowly, for example at idle speed; and to provide starting power. The battery, at times, is disconnected, for example for maintenance, replacement, or the like. In case of carelessness, it is possible that the battery, upon re-attachment, is reversely polarized. Reverse polarization, even if for only a very short period of time, causes severe damage to the semiconductor rectifiers with which alternators, providing power to the battery, are customarily connected. Reverse polarization can occur, for example, by reverse positioning of the battery in the battery well, interchange of connectors, use of a replacement battery with differently positioned terminals or the like. If the battery is connected with reverse polarization, the diodes, at least of the rectifier, are usually damaged to the resulting high current which will flow. It has occurred that users of such battery charge systems, as well as careless installers, will not admit that the battery was reversely connected and insist with respect to the manufacturer of the rectifier that the rectifier diodes were originally defective. In such cases, and to prevent false warranty claims, it is important to be able to prove to the user that the battery was reversely connected.
It has been tried to fit into the rectifier system weak points, fuses, or the like, using thin contact wires, connections which are weakened, or of restricted diameter in the bridge circuits of the rectifiers, and burn-through fuses. All such elements have been found unsatisfactory since they can respond even upon normal use of the rectifier. Particularly in automotive vehicles, the stress placed on such "weak links" is considerable, for example due to vibration, corrosion, and the like, causing interruption of the rectifier circuit, and failure of the power supply. Additionally, it has been found that fuses have to be dimensioned to be able to carry high currents in order to carry the normally high currents which may arise upon substantial charging. Such high-current fuses would not respond quickly enough upon reverse polarization to prevent damage to the rectifier. Fuses and weak links if designed to carry high charging currents, such as fusible links, are not sufficiently reliable to permit response to the high reversely polarized currents, although only practically instantaneous by occurring.